1. Field of the Invention
This invention concerns increasing the yield while at the same time increasing the production potential of reactors for producing urea, formed by synthesis between ammonia and carbon dioxide at high pressure and temperature. The reactors include an internal space for cocurrent fluid flow by a liquid phase and a gas phase, the internal space being divided into compartments to avoid excessive mixing of the liquid phase and to allow for intermittent redistribution of gas in bubbles of a size suitable for increasing heat and matter exchange between the two phases.
The invention also concerns a device including seven perforated flat baffles or plates distributed transversely in a cylindric reactor shell for creating compartments therein, through which flow gas and liquid containing urea.
2. Description of the Related Art
In modern reactors for the synthesis of urea, two phase (a gas phase and a liquid phase) flow cocurrently inside the cylindrical shell of the reactor (which is under pressure). The shell is divided into compartments by seven perforated plates. The purpose of this configuration is to avoid excessive mixing of the entire liquid phase contained in the reactor, which would tend to turn it into a complete mixing reactor, thus reducing the urea yield. The amount of mixing is reduced by dividing the reactor into several stages by means of seven plates. The behavior of the liquid phase approaches that of the behavior of a piston flow reactor, which is notoriously the most favorable for keeping the urea yield relatively high.
By dividing the reactor into seven stages by means of perforated plates, it is also possible to redistribute the gas which flows upwards along the column intermittently in smaller bubbles more suitable for increasing heat and matter exchange between the two phases. In effect, the rising showers of bubbles are subjected to coalescence phenomena which progressively increase the size of the bubbles, thus reducing the exchange surface between phases. This negative phenomena is partly compensated for by the redistribution brought about by the perforated flat plates.
However, the cocurrent flow of gas and liquid over each perforated plate produces some adverse effects on both the heat and matter exchange and the urea yield. The latter is due to a diminution of the flow of reagents into the liquid phase, as well as to the reduction in temperature because of the smaller exothermic reaction and also to the reduction in the liquid hold-up in the reactor. In effect, steam and liquid cannot go through the perforations in the flat baffles simultaneously, but are forced to do so alternately by means of forming showers of steam bubbles, separated by liquid pistons in continuous phase. Such an arrangement, as compared with a uniform distribution of the bubbles with the same amount of steam brings about a higher concentration of bubbles inside the showers alternating with the liquid pistons. The result is a significant increase in coalescence of the bubbles between one plate and the next (increasing their average size), a reduction of the steam/liquid surface, and a worsening of the gas phase/liquid phase exchange since less steam is exchanged with the liquid phase. The volume available for this phase is also reduced (and the temperature it has reached is also lowered). Moreover, between the plates and the cylinder there is in general a circular slit through which part of the steam may go with less exchange efficiency. All this causes a reduction of the urea yield, compared with the yield obtainable with a uniform distribution.
Also, the possibility of increasing the production potential in reactors in existing plants is limited. Generally, the reactor, due to its potential liquid phase capacity, is very large compared to the nominal urea production required of it, and this fact would lend itself, in principle, to possible increases in production with an almost constant urea yield. However, urea yield drastically worsens as the gas and liquid capacity increases, because of the poor distribution of gas, the size of the bubbles due to parasitical coalescence, the matter and heat exchange between phases, the effective volume left to the liquid phase (in which the reaction forming the urea takes place). Accordingly, urea production does not increase proportionately to the increase in total liquid and gas capacity.